Marker entities should be able to appear on the locator bar

The Marker entity type needs to be added to the list of entity types that can act as waypoints on the locator bar.

The locator bar added in 1.21.6 can show other entities if the relevant NBT data is set up on the entity. However, the Marker entity used by mapmakers specifically to mark locations cannot have the relevant NBT tags set. This means that to have a waypoint set to appear on the locator bar we have to revert to using an invisible armour stand, which is what the marker entity was added to replace.